Active filter comprising a transconductor circuit with high-linearity differential input

Abstract: The invention relates to a transconductor circuit with a differential input and a single output, comprising two input transistors (M1, M2) whose primary conduction terminals (D1, S1, D2, S2) are respectively connected together; in this way, variations in load current and voltage can be made lower, thereby also lowering distortion from changes in their transconductance.

Description: This invention relates to an active filter comprising at least a differential input transconductor circuit according to the preamble of Claim 1. Transconductor circuits are well known, and recognized is their prerequisite for linearity as the amplitude of the input voltage varies. Patent Application GB-A-2 175 763 addresses, inter alia, this problem and proposes as a preferred embodiment the half-circuit shown in Figure 1. This comprises a MOS transistor denoted by M which has its source terminal connected to a reference, usually ground, potential GND and its gate terminal connected to the circuit input; and a BJT transistor having an emitter terminal connected to the drain terminal of the transistor M and a base terminal connected to a reference bias potential UDC; a bias current IDC is flowed through the collector of the transistor Q. The transconductance G of the circuit is given by, G = K * H * VDS where, VDS is the drain-to-source voltage of the transistor M, K is a coefficient dependent on the manufacturing process used for the transistor M and on the gate-to-source voltage VGS, and H is a coefficient dependent on the transistor M geometry. In order to limit the distortion due to the coefficient K, the aforementioned application proposes the use of differential circuits formed of fully symmetrical half-circuits. To limit the distortion due to the presence of the term VDS, the aforementioned Application proposes, as a preferred embodiment, that the transistor Q be used to lower the output impedance as seen from the transistor M on the drain terminal, as shown in the half-circuit of Figure 1; in fact, when the voltage VGS of transistor M varies, its (output) drain current also varies, and consequently, so does the (output) voltage on the load applied to the drain terminal, which corresponds to the term VDS. As second choice, it proposes that the transistor Q be replaced with an output stage consisting of a feedback connected circuit which has a much lower input impedance and much higher output impedance. Such an output stage is inherently fairly complicated circuit-wise, and becomes more so where several transconductor circuits must be used, as is the case with active filters, and especially if they are to be integrated to a chip. The feedback scheme, moreover, restricts the structure utility range by placing limits on its frequency. Also known from the patent application No. GB-A-2 175 763 is a transconductor circuit, having differential input and a single output and comprising first and second MOS transistors having drain and source terminals connected to each other and gate terminals receiving differential input, as well as a bipolar transistor, having the base terminal connected to a bias voltage reference. It is the object of this invention to provide a solution to the problem of the most straightforward linearity circuit-wise, which solution can suit particularly well highfrequency active filters. This object is achieved by an active filter having the characterizing features of Claim 1. Further advantageous aspects of this invention are set forth in the subclaims. By using a differential type of transconductor wherein the two input transistors have output terminals connected together, the effect of variations in the output current, and ensuing distortion, can be greatly reduced. This solution is specially effective where the signal voltages to the differential inputs are approximately equal in modulo and offset by about 180 degrees. Understandably, best performance would be achieved with input transistors which are as far as possible identical. The invention can be more clearly understood by having reference to the following description, to be read in conjunction with the accompanying drawings, in which: Figure 1 is a circuit diagram of a conventional transconductor circuit; Figure 2 is a circuit diagram of a transconductor circuit comprised in the active filter according to the invention; Figures 3a and 4a show, in block diagram form, conventional active filters; and Figures 3b and 4b show, also in block diagram form, active filters according to the invention which correspond to those illustrated in the respective Figures "a". The transconductor circuit, shown in Figure 2, has a differential input on first I1 and second I2 input terminals and a single output on an output terminal OP; it comprises a first input transistor M1 of the MOS type and a second input transistor M2, also of the MOS type, which have their gate terminals, G1 and G2, respectively connected to the input terminals I1 and I2, source terminals S1 and S2 connected together and to a first node ND1, and drain terminals D1 and D2 connected together and to a second node ND2; it further comprises a third transistor QO of the BJT type which has its emitter terminal E connected to the node ND2, base terminal B connected to a reference bias potential UDC, and collector terminal C connected to a third node ND3. In addition, a bias current generator GDC is connected to the node ND3, with the output terminal OP being also connected to the node ND3. The potential at the node ND2 can be controlled through the transistor QO. With the above assumption, and neglecting the bias signals, it is easily seen that the signal current being flowed through the transistor QO will be small, so that small will also be variations caused by the base-to-emitter voltage VBE. In fact, this current is given by the signal voltage on the input I1 multiplied by the transconductance of the transistor M1 plus the signal voltage on the input I2 multiplied by the transconductance of the transistor M2. The node ND2 potential is, therefore, quite constant, and the drain-to-source voltage VDS of the transistors M1, M2 and, hence, the transconductance variations, will be limited. This results in distortion being low. The circuit shown in Figure 2 lends itself to several modifications, among which are the transistors M1, M2 and QO, which may either be of the MOS tyke or the BJT type. The node ND1 is usually held at a predetermined fixed potential, often at ground potential. The node ND1 could be connected, however, to another bias current generator. The assumption made (signal voltages to the differential inputs approximately equal in modulo and offset by about 180 degrees) is frequently met in actual practice. A conventional active filter of the first order is shown in Figure 3a to comprise two transconductor circuits TT of a traditional type connected in series. The center tap of that link is connected to ground through a capacitor CC and to directly to the filter output, thereby forming a feedback path. An active filter according to the invention which is implemented with a transconductor TD according to Figure 2, is shown in Figure 3b. The transconductor TD has a first input connected to the filter input, and a second input connected to the output of the transconductor TD; this output is connected to ground through a capacitor CC and to the filter output by a direct link. The circuit of Figure 3b illustrates that the assumption has been met, as a first approximation, at least at frequencies below the circuit cutoff frequecy. Figure 4a shows an active filter of the second order implemented with transconductors TT of conventional design, a first capacitor C1 and a second capacitor C2. It is essentially the equivalent of having two filters of the first order, similar to the one shown in Figure 3a, feedback cascade connected. Figure 4b shows an active filter according to the invention of the second order and implemented with transconductors TD according to Figure 2. This corresponds essentially to having two filters of the first order like that shown in Figure 4a cascade connected and fed back.

Claims: An active filter comprising at least a transconductor circuit having differential input on first (I1) and second (I2) input terminals and a single output on an output terminal (OP), said transconductor circuit comprising: first (M1) and second (M2) input transistors connected between a first (ND1) and a second node (ND2) and having control terminals (G1, G2) respectively connected to the input terminals (I1, I2), said first (M1) and second (M2) input transistors having two first homolog terminals (S1, S2) connected together and to the first node (ND1) and two second homolog terminals (D1, D2) connected together and to the second node (ND2). a current generator (GDC) connected to a third node (ND3), in turn connected to the output terminal (OP), and a third transistor (QO) connected between said second (ND2) and third node (ND3) and having a control terminal (B) connected to a reference bias potential (UDC), thereby controlling the potential at said second node (ND2); said active filter further comprising at least one capacitor (CC) with one terminal connected to a reference potential and the other terminal connected to the output terminal (OP) of the transconductor circuit characterised in that one of said input terminals (I1, I2) is directly connected to the output terminal (OP) to form a feedback circuit. An active filter according to Claim 1, characterized in that said first and second transistors (M1, M2) of the transconductor circuit are of the MOS type. An active filter according to either Claim 1 or 2, characterized in that said third transistor (QO) of the transconductor circuit is of the BJT type. An active filter according to either Claim 1 or 2 or 3, characterized in that said first node (ND1) of the transconductor circuit has a predetermined fixed potential. An active filter according to either Claim 1 or 2 or 3, characterized in that the transconductor circuit comprises an additional bias current generator connected to said first node (ND1).

Inventors: BASCHIROTTO ANDREA, CASTELLO RINALDO, REZZI FRANCESCO, BASCHIROTTO, ANDREA, CASTELLO, RINALDO, REZZI, FRANCESCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H03F3
IPC codes: H03F3, H03H11